Worker threads can trigger MaxListenersExceededWarning when stdout/stderr are overloaded or corked

[loganfsmyth]

• Version: 15.10.0
• Platform: macOS
• Subsystem: worker

What steps will reproduce the bug?

Run node example.js with these files:

example.js

const { Worker } = require("worker_threads");

process.stdout.cork();
for (let i = 0; i < 15; i++) {
  new Worker("./example_worker.js");
}
process.stdout.write(Buffer.alloc(process.stdout.writableHighWaterMark, "!"));

example_worker.js

console.log("OUT");

How often does it reproduce? Is there a required condition?

Reproduces consistently.

What is the expected behavior?

Should run and then exit silently.

What do you see instead?

Runs and logs a warning:

(node:23883) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 drain listeners added to [WriteStream]. Use emitter.setMaxListeners() to increase limit
(Use `node --trace-warnings ...` to show where the warning was created)

Additional information

The worker logic uses .pipe() and tries to avoid issues like this with its pipeWithoutWarning helper, but that doesn't apply to events that are added internally within pipe at a later time, so that approach just doesn't seem right.

.pipe() adds dest.on("drain", ... when the destination is corked or overloaded, and so if the number of workers is > defaultMaxListeners and the destination write returns false, Node logs a warning.

[targos]

@nodejs/workers

[jasnell]

@addaleax ... any thoughts on this one? Really the only "fix" I can imagine makes sense here is to increase the max listener count that triggers the warning.

[addaleax]

Isn’t this more of a streams issue, rather than Worker-specific? I don’t think this warning should occur when adding piping from more than 10 streams into the same output stream.

That being said, if everybody’s good with the solution of just setting the max listener count for the stdio streams to Infinity by default, that’s also great.